Atomic force microscopy of scanning and image processing

ABSTRACT

A topographic profile of a structure is generated using atomic force microscopy. The structure is scanned such that an area of interest of the structure is scanned at a higher resolution than portions of the structure outside of the area of interest. An profile of the structure is then generated based on the scan. To correct skew and tilt of the profile, a first feature of the profile is aligned with a first axis of a coordinate system. The profile is then manipulated to align a second feature of the profile with a second axis of the coordinate system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of Ser. No. 11/413,579 filed Apr. 28,2006 for “ATOMIC FORCE MICROSCOPY OF SCANNING AND IMAGE PROCESSING”.

BACKGROUND

The present invention relates to scanning probe microscopy, and moreparticularly to using atomic force microscopy to produce an imageprofile representative of a structure.

Atomic force microscopy (AFM) is a metrology technique that is usefulfor measuring and imaging surface features of structures havingdimensions in the nanometer and micrometer range. AFM may be used toscan structures made of any material in a short period of time toproduce high resolution two-dimensional and three-dimensional images ofthe structure. AFM is an important tool for measuring dimensions ofdevices in the semiconductor industry, including magnetic recordingdevices and microelectromechanical system (MEMS) devices.

The lateral resolution of an image produced from an AFM scan of astructure is defined by the scan area size and the number of pixels inthe image. Thus, in order to increase the lateral resolution of animage, the size of the scan area may be reduced or the amount of data inthe image (i.e., the number of pixels) may be increased. However, areduction in the size of the scan area removes contextual details aroundthe scanned area of interest, which makes determining the relative sizesand positions of features within the structure difficult. On the otherhand, to increase in the amount of data in the scan, the scan speed maybe reduced, which decreases measurement throughput and may result indrift errors in the image. The increased amount of data in the scan alsowastes the limited available data on areas outside of the areas ofinterest in the scan.

In addition, the small dimensions of the scanned structure result inmissed details or the introduction of artifacts into the resultingimage. For example, when scanning a structure including features havingsignificant topographical transitions, feedback overshoot may occur atthe transition locations, resulting in lost details in therepresentative image at the transition locations. In addition, a scan ofa flat or planar feature in the structure may result in a curving orbowing artifact in the resulting image at the location of the flat orplanar feature. This may be caused by the relative sizes and shapes ofthe scanning probe tip and the scanned feature. Image curvature may alsooccur when the scanning probe tip moves faster in one direction than theother along the structure surface because environmental vibrations,thermal drifting, and air flow along the probe tip may affect the imagein the slower scan direction.

SUMMARY

The present invention relates to the generation of a topographic profileof a structure using atomic force microscopy. The structure is scannedsuch that an area of interest of the structure is scanned at a higherresolution than portions of the structure outside of the area ofinterest. An image of the structure is then generated based on the scan.To correct skew and tilt of the image, a first feature of the image isaligned with a first axis of a coordinate system. The image is thenmanipulated to align a second feature of the image with a second axis ofthe coordinate system. In one aspect, curvature artifacts are thenremoved from the image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an atomic force microscope probepositioned over a surface of a structure.

FIG. 2 is a perspective view of a cavity transition feature formeasuring with atomic force microscopy (AFM) techniques.

FIG. 3A is a schematic view of a scanning method for increasing theamount of data at an area of interest on a structure.

FIG. 3B is a schematic view of another scanning method for increasingthe amount of data at an area of interest on a structure.

FIG. 4A is a two-dimensional plot of a raw scanned image profile of acavity transition feature.

FIG. 4B is a two-dimensional plot of a tilted image profile of thecavity transition feature.

FIG. 4C is a three-dimensional plot of the tilted image profile shown inFIG. 4B.

FIG. 4D is a three-dimensional plot of a true image profile of thecavity transition feature.

FIG. 5A is a schematic view of a first scan for use in correcting imagecurvature artifacts in an AFM scan.

FIG. 5B is a schematic view of a second scan for use in correcting imagecurvature artifacts in an AFM scan.

FIG. 6 is a flow diagram showing a process for correcting imagecurvature artifacts in an AFM scan.

FIG. 7 is a flow diagram showing steps for correcting image curvatureartifacts using a master reference scan.

DETAILED DESCRIPTION

FIG. 1 is a perspective view an atomic force microscope 10 positionedover a surface of structure 12. Atomic force microscope 10 includesprobe 11 having cantilever portion 14 and tip portion 16. Atomic forcemicroscope 10 also includes light source 18, position sensitive detector20, and processor 22. Light source 18 emits a beam 24 that is reflectedby cantilever 14 and received by position sensitive detector 20.Processor 22 receives signals from position sensitive detector 20 andprovides signals to control movement of probe 11 relative to structure12.

Structure 12 is the pole tip region of a magnetic recording system,including slider 26 carrying reader structure 28 and writer structure30. The atomic force microscopy (AFM) techniques described herein areuseful for measuring and imaging feature characteristics of structure12, such as pole tip recession (PTR) features of reader structure 28 andwriter structure 30. It should be noted that structure 12 is shownmerely for purposes of illustration, and the AFM techniques describedherein are also useful for measuring and imaging nanometer andmicrometer scale surface features of other structures. For example, theAFM techniques may also be used to measure feature characteristics inother magnetic recording device structures, such as a cavity transitionfeature as shown in FIG. 2.

Atomic force microscope 10 measures physical characteristics orproperties of structure 12, such as feature dimensions and surfacefinish. Probe tip 16 is positioned in very close proximity (i.e., withinpicometers) to the surface of structure 12 to allow measurements ofstructure 12 over a small area. Probe tip 16 is moved relative tostructure 12 using extremely precise positioning. For example, processor22 may control motion of probe 11 such that probe tip moves along thesurface of stationary structure 12. Alternatively, processor 22 maycontrol a device such as a tube scanner to move structure 12 while probe11 remains stationary. As probe tip 16 moves over the surface ofstructure 12, features on the surface of structure 12 cause cantilever14 to bend in response to the force between probe tip 16 and structure12.

A position detector measures the amount of deflection in cantilever 14,which may be used to generate an image representation of structure 12.In particular, light source 18 (e.g., a laser) reflects light beam 24off of cantilever 14 to position sensitive detector 20. Positionsensitive detector 20 may include two side-by-side photodiodes such thatthe difference between the signals generated by the photodiodesindicates the position of light beam 24 on position sensitive detector20, and thus the angular deflection of cantilever 14. Because thedistance between cantilever 14 and position sensitive detector 20 isgenerally thousands of times the length of cantilever 14, the motions ofprobe tip 16 are greatly magnified.

FIG. 2 is a perspective view of slider 26 including air bearing surface(ABS) 42, transition edge 43, cavity transition 44, and cavity 46.Cavity transition 44, which may be defined using ion milling techniques,has very small features varying from nanometer to micrometer scalesizes. Measurement of the features of cavity transition 44 is importantin various aspects in the development of the device, including designimprovement, device model validation, and device performanceenhancement. For example, in a magnetic recording device, themeasurement of cavity transition 44 is important for understandingflying performance of slider 26. The AFM techniques described herein maybe used to measure the properties of cavity transition feature 44.

Variable Scan Data Density

The lateral resolution of an image produced from a scan of a structureis defined by the scan area size and the number of pixels in the image.Conventionally, atomic force microscope 10 moves relative to thestructure at a constant speed, and the position of probe 11 isperiodically sampled by processor 22. The resulting image has a uniformresolution across the entire scanned region.

Some structures may include a region or area of interest having a targetfeature or characteristic of which a more detailed scan may be desired.For example, a detailed scan of cavity transition 44 of slider 40 shownin FIG. 2 may be desirable for precise measurement of the dimensions andother characteristics of the transition profile. In order to increasethe resolution in the area of interest, the number of pixels or datapoints in the area of interest may be increased. Because the number ofpixels available for a given scan is often fixed, the resolution in thearea of interest is thus increased at the expense of limited views ofthe areas surrounding the area of interest. However, it is alsoimportant to maintain the contextual details around the area of interestsuch that the relative sizes and positions of features within the areaof interest are more easily determinable.

FIG. 3A is a schematic view of an approach for increasing the amount ofdata at an area of interest on a scanned structure while maintaining thecontextual details of the surrounding areas. In FIG. 3A, the scan area50 of a structure is shown including area of interest 52. Probe 11 movesaround scan area 50 in scan pattern 54 during image acquisition inresponse to control signals from processor 22. In particular, scanpattern 54 is programmed in processor 22, and processor 22 controlsmovement of probe 11 relative to the structure in the programmed scanpattern 54. As probe 11 is moved relative along scan pattern 54 withinscan area 50, data points or pixels 56 are sampled by processor 22. Thatis, processor 22 periodically communicates with position sensitivedetector 20 to receive information about the position of probe tip 16relative to position sensitive detector 20. This information is used byprocessor 22 to set locations of data points 56, which correspond to thepixels in the image generated from data points 56 by processor 22. Theresulting image is representative of the structure includes graphicalrepresentations of the surface features and characteristics of thestructure.

Processor 22 samples data points 56 in the portions of scan area 50outside of area of interest 52 as probe 11 moves along the y-directionrelative to the structure. The data points 56 are separated by adistance d_(y1) in the y-direction and a distance dx1 in the x-directionin areas outside of area of interest 52. In one embodiment, distanced_(x1) and distance d_(y1) are equal to provide a continuous lateralresolution in the portions of scan area 50 outside area of interest 52.

In order to increase the resolution at area of interest 52, scan pattern54 is programmed such that more data points 56 are sampled in area ofinterest 52 during the scan than in the portions surrounding area ofinterest 52. The programmed location of area of interest 52 in scanpattern 54 may be determined during the scan based on known positioninformation on the scanned structure, or based on surrounding featurecharacteristics sensed by probe tip 16. When probe tip 16 is close toarea of interest 52, processor 22 reduces the distance between adjacentscan lines in the x-direction to distance d_(x2) to increase the densityof data points 56 in the x-direction (i.e., probe tip 16 moves a smallerdistance relative to the structure between adjacent scan lines). Whenprobe tip 16 is in area of interest 52, processor 22 increases thenumber of data points 56 sampled along each scan line (i.e., decreasesthe spacing between each data point 56 to d_(y2)), which increases thedata density in area of interest 52 in the y-direction. The resolutionin the y-direction may be increased by, for example, increasing the rateat which processor 22 samples the position information of cantilever 14from position sensitive decoder 20, by adjusting the rate at which probe11 is moved relative to the structure, or a combination of increasingthe sample rate and decreasing the relative motion between probe 11 andthe structure. In the embodiment shown, the resolution of the scan inarea of interest 52 is three times that in the portions of scan area 50surrounding area of interest 52.

Atomic force microscope 10 allows the density of data points 56 to beadjusted during the scanning process. From a single scan, the resultingimage of the structure has a higher resolution in area of interest 52than in the remainder of scan area 50. This scan process not onlypreserves the contextual details in the areas around area of interest52, but also allows for greater throughput of scans and measurements ofthe structure since multiple scans are not required.

It should be noted that scan pattern 54 is merely illustrative, andother scan patterns may be used for imaging a structure having differentcharacteristics. For example, if a structure includes multiple areas ofinterest, the scan pattern may be programmed to increase the samplingrate or reduce the scan speed at the multiple areas of interest toincrease the resolution in those areas. In addition, scan pattern 54 mayinclude multiple levels of resolution within the same scan area 50.

FIG. 3B is a schematic view of an alternative approach for increasingthe amount of data at area of interest 52 while maintaining thecontextual details of the surrounding areas in scan area 50. In thisembodiment, multiple scans having different resolutions are combined toproduce an image profile having a higher resolution at the area ofinterest than in the surrounding areas of the scan area.

A first scan is performed in a scan pattern 60 across the scan area 50at a first data density. In particular, processor 22 samples data points62 in scan area 50 as probe 11 moves in the x-direction and y-directionrelative to the structure. The data points 62 are separated by adistance d_(x1) in the x-direction and a distance d_(y1) in they-direction. In some embodiments, data points 62 are evenly distributedthroughout scan area 50. The resulting scan pattern 60 thus provides arelatively low resolution sampling of scan area 50.

A second scan is performed in a scan pattern 64 at a second data densityhigher than the first data density in area of interest 52. Inparticular, processor 22 samples data points 66 in area of interest 52as probe 11 moves in the x-direction and y-direction relative to thestructure. The data points 66 of the second scan are separated by adistance d_(x2) in the x-direction and a distance d_(y2) in they-direction. In order to increase the data density within area ofinterest 52, distances d_(x2) and d_(y2) are smaller than thecorresponding distances d_(x1) and d_(y1) of the first scan. In theembodiment shown, the data density of the second scan is three timesgreater than the data density of the first scan.

The two scans are then integrated by aligning common data points 68 thatare shared between scan pattern 60 and scan pattern 62. For example,during the scanning process, processor 22 may record the location ofeach data point sampled. The scans would then be integrated together bymatching locations of data points 62 in scan pattern 60 with data points66 in scan pattern 64. Processor 22 may also record topographicalcharacteristics of each data point sampled, which would allow the scanpatterns to be integrated by matching the topographical pattern of scanpattern 60 with that of scan pattern 62. In any case, when the scanshave been integrated, the resulting image of scan area 50 has a higherresolution in area of interest 52 than in the remainder of scan area 50.

It should be noted that scan patterns 60 and 64 are merely illustrative,and other scan patterns may be used for imaging a structure havingdifferent characteristics. For example, if a structure includes multipleareas of interest, additional scans may be performed to produce a scanpattern for each area of interest for ultimate integration into thecontextual scan pattern. In addition, scan pattern 54 may includemultiple levels of resolution within the same area of interest 52. Thatis, multiple scans may be taken of area of interest 52 with varying datadensities such that the most relevant or interesting portions of area ofinterest 52 have the highest data density, and the data densitydecreases with increasing distance from area of interest 52.

Transition Profile Skew Correction

When a structure (such as slider 26) is scanned by atomic forcemicroscope 10, it is held in position on a linear stage or otherpositioning device, such as in a tray, by a fixture, or with adhesive.However, due to positioning errors, the structure may not be preciselyaligned with atomic force microscope 10 with respect to the contours ofthe programmed scan pattern. For example, the structure may be skewed inone direction relative to the scan pattern, or the structure may betilted relative to the plane of the scan pattern. The image resultingfrom the misaligned scan thus may not represent the true profile of thestructure, making an accurate measurement of the dimensions of thescanned structure and features of the structure difficult. In addition,even if the positioning of the structure relative to atomic forcemicroscope 10 is perfect, variations in the components of atomic forcemicroscope 10 (e.g., due to environmental conditions) may result in amisaligned image.

For example, in a scan of slider 26 (FIG. 2), the dimensions andcharacteristics of cavity transition 44 may be measured. Measurement ofcavity transition 44 is important for understanding flying performanceof slider 26, as well as for device design improvement, modelvalidation, and performance enhancement. Cavity transition 44 may bemeasured relative to another feature on slider 26, such as ABS 42, toallow for analysis of the shape of cavity edge 43 and cavity transition44. However, if the image of ABS 42 is skewed or tilted due tomispositioning of slider 26 or due to performance of atomic microscope10, characteristics of cavity transition 44 may be difficult to measure.

FIGS. 4A-4D show an approach to correcting the skew and tilt in an imagebased on a scan of slider 26 including cavity transition feature 44. Tominimize alignment artifacts caused by variations in the components ofatomic force microscope 10 and environmental conditions, the scan wasperformed at a 0.2 Hz sampling rate with probe tip 16 having a radius ofless than about 30 nm proximate to slider 26. Each step described hereinwith regard to correcting the skew and tilt in the image may beperformed by processor 22 or by a microprocessor based system externalto atomic force microscope 10.

FIG. 4A is a two-dimensional plot of a raw scanned image profile basedon a scan of slider 26 including cavity transition 44 under the aboveconditions. The contour lines shown in the plot in FIG. 4A representheight changes in slider 26 relative to the z-axis. For context,transition edge 43 and cavity transition 44 are labeled in the image. Asis shown, the image representative of slider 26 is skewed relative tothe y-axis. In addition, the height change relative to the z-axis shouldbe more pronounced at cavity transition 44, and ABS 42 should include nocontour lines because it should be co-planar with the x-axis and they-axis. However, because few contour lines are shown at cavitytransition 44 and several contour lines are shown around ABS 42, theimage is also tilted relative to the desired orientation (i.e., with ABS42 parallel with the xy-plane). The correction of the tilt in the imagewill be described with regard to FIGS. 4C and 4D.

The skew in the two-dimensional view of slider 26 may be corrected bychoosing a feature in the scan and re-orienting the image based on thatfeature. For example, a derivative map of the image (i.e., a plot of thederivative at every location in the image) shown in FIG. 4A may begenerated to determine the locations of transition features (e.g.,transition edge 43) in slider 26. The transition feature may then beused as an alignment index for aligning the image within thetwo-dimensional view. FIG. 4B shows the two-dimensional image of slider26 including cavity transition 44 after aligning the image relative tothe x-axis and y-axis based on transition edge 43.

FIG. 4C is a three-dimensional plot of the tilted image profile ofslider 26. As can be seen, while transition edge 43 is aligned with they-axis, ABS 42 is tilted relative to the xy-plane. To facilitatemeasurement of cavity transition 44, the image of slider 26 may berotated relative to the xy-plane. While this rotation may be performedin Cartesian coordinates, the rotation is simplified by first convertingeach data point in the image of slider 26 to spherical coordinates.Thus, for each data point having coordinates (x, y, z), thecorresponding spherical coordinates (R, θ, φ) are given by

$\begin{matrix}{{R = \sqrt{x^{2} + y^{2} + z^{2}}},} & \left( {{Equation}\mspace{20mu} 1} \right) \\{{\theta = {\arctan\left( \frac{y}{x} \right)}},{and}} & \left( {{Equation}\mspace{20mu} 2} \right) \\{{\phi = {\arctan\left( \frac{z}{\sqrt{x^{2} + y^{2}}} \right)}},} & \left( {{Equation}\mspace{20mu} 3} \right)\end{matrix}$where R is distance from the origin to the data point, θ is the anglefrom the xz-plane to the point, and φ is the angle from the xy-plane tothe point.

The image of slider 26 may be rotated relative to the xy-plane to levelABS 42 by offsetting the angle φ by a correction angle α based on theslope of the tilted ABS 42,

$\begin{matrix}{{\alpha = {\arctan\left( \frac{\mathbb{d}y}{\mathbb{d}x} \right)}},} & \left( {{Equation}\mspace{20mu} 4} \right)\end{matrix}$where dy/dx is the slope of tilted ABS 42 relative to the xy-plane. Torotate the image of slider 26, each data point may be offset bycorrection angle α and converted from spherical coordinate back toCartesian coordinates. Thus, for each data point having coordinates (R,θ, φ−α), the corresponding Cartesian coordinates (x, y, z) are given byx=R cos(φ−α)cos(θ)  (Equation 5),y=R cos(φ−α)sin(θ)  (Equation 6), andz=R sin(φ−α)  (Equation 7).A plot of the three-dimensional image after rotation, which reflects thetrue profile of the slider 26 and cavity transition 44, is shown in FIG.4D.

Artifact Curvature Correction

During an AFM scan, probe tip 16 moves along the scanned surface fasterin one direction than in the other direction. For example, as shown inFIG. 3A, probe tip 16 samples the surface along scan lines that runalong the y-direction. Thus, for each scan line, a group of data points56 is sampled in the y-direction, while only a single data point issampled in the x-direction. In the fast scan direction, probe tip 16 maytake a few seconds or less to move from one end of the scan area to theother, while it may take as long as several minutes to move from one endof the scan area to the other in the slow scan direction. Withoutcorrection, environmental vibrations, machine drifting, and airflowalong probe 11 can cause curvature artifacts in the image in the slowscan direction.

FIGS. 5A and 5B are schematic views of scans that may be used forcorrecting image curvature artifacts in an AFM scan. FIG. 6 is a flowdiagram showing a process for correcting image curvature artifacts in anAFM scan. The scan shown in FIG. 5A is performed within a scan area(step 100), and the scan shown in FIG. 5B is performed in the same scanarea at 90° with respect to the first scan (step 102). For example, thescans shown may be performed within scan area 50 in FIGS. 3A and 3B. Thescan patterns shown in FIGS. 5A and 5B are representative of any type ofscan pattern having a fast scan direction and a slow scan direction, andare not necessarily representative of an actual scan pattern.

In FIG. 5A, the scan area can be represented by a matrix of data pointsa_(xy) (x, y=1˜n, where n is the periphery dimension of the scan area).For simplicity, the scan area shown is square, but the curvatureartifact correction method described is also applicable to scan areashaving other dimensions and characteristics. The fast scan direction isthe x-direction and the slow scan direction is the y-direction. The fastscan direction has an average profile ā_(x) and the slow scan directionhas an average profile ā_(y) (step 104), where

$\begin{matrix}{{{\overset{\_}{a}}_{x} = {\frac{1}{n}{\sum\limits_{y = 1}^{n}a_{xy}}}},{and}} & \left( {{Equation}\mspace{20mu} 8} \right) \\{{\overset{\_}{a}}_{y} = {\frac{1}{n}{\sum\limits_{x = 1}^{n}{a_{xy}.}}}} & \left( {{Equation}\mspace{20mu} 9} \right)\end{matrix}$

Similarly, in FIG. 5B, the scan area can be represented by a matrix ofdata points b_(xy) (x, y=1˜n, where n is the periphery dimension of thescan area). In this scan, the fast scan direction is the y-direction andthe slow scan direction is the x-direction. The fast scan direction hasan average profile b _(y) and the slow scan direction has an averageprofile b _(x) (step 104), where

$\begin{matrix}{{{\overset{\_}{b}}_{x} = {\frac{1}{n}{\sum\limits_{y = 1}^{n}b_{xy}}}},{and}} & \left( {{Equation}\mspace{20mu} 10} \right) \\{{\overset{\_}{b}}_{y} = {\frac{1}{n}{\sum\limits_{x = 1}^{n}{b_{xy}.}}}} & \left( {{Equation}\mspace{20mu} 11} \right)\end{matrix}$

The average profiles along the fast scan direction in the two scans,ā_(x) and b _(y), represent the true profile of the structure, while theaverage profiles along the slow scan direction, ā_(y) and b _(x), are acombination of the true profile of the structure and curvature artifactsdue to drifting of atomic force microscope 10 and other environmentaleffects. It is difficult to separate the true shape and driftingartifacts in the slow scan direction. Thus, all information in the slowscan direction may be removed using zero order image flattening bysetting the mean height of each scan line in the fast scan direction tozero (i.e., setting the average profile in the slow scan direction tozero) (step 106). The flattened images may thus be represented bya′_(xy) and b′_(xy), where ā′_(y)=0 and b′_(x)=0, anda′ _(xy) =a _(xy) −ā _(y)  (Equation 12), andb′ _(xy) =b _(xy) − b _(x)  (Equation 13).

The correct profile along the slow scan direction for each scan can beobtained by setting the mean height of each fast scan line according tothe average profile along the fast scan direction of the other scan(step 108). The corrected images may thus be represented by and, wherea″ _(xy) =a′ _(xy) + b′ _(y)  (Equation 14), andb″ _(xy) =b′ _(xy) +ā′ _(x)  (Equation 15).

While the two images represented by a″_(xy) and b″_(xy) may besubstantially identical, small differences may exist due to variationsin the performance of atomic force microscope 10. Thus, to obtain themost accurate representation of the true profile of the structure,

$\begin{matrix}{c_{xy} = {\frac{1}{2}{\left( {a_{xy}^{''} + b_{xy}^{''}} \right).}}} & \left( {{Equation}\mspace{20mu} 16} \right)\end{matrix}$

Master Reference Subtraction

At micrometer-level scan lengths, bowing can occur in traditional tubescanner atomic force microscopes, which may also produce curvature inthe resulting image. The amplitude and shape of the bowing vary betweenatomic force microscopes, and may change with aging, temperature, andhumidity. Positional offsets between scans of the same surface in thesame scan area may also vary the curvature in the corresponding image.

To avoid the contribution of bowing to measurement error, a referencescan may be taken on a flat surface with the same scan settings (e.g.,scan size and offsets) of a regular scan. The reference scan issubsequently subtracted from the regular scan to obtain an image withoutcurvature due to the bowing effect. However, any real curvature in thesurface of the reference scan will be added to the measurement results.In addition, scan defects, irregular scan lines, and particlecontamination in the reference scan may add error to the measurementresults.

FIG. 7 is a flow diagram showing steps for correcting image curvatureartifacts due to the bowing effect using a master reference scan. Amaster reference scan is generated with atomic force microscope 10 byscanning a flat surface (step 120). For example, the flat air bearingsurface of slider 26 in FIG. 1 may be used to generate the masterreference scan. The master reference scan is performed under conditionsso as to minimize defects in the reference scan. In particular, atomicforce microscope 10 is used in environmental conditions that minimizebowing effect and prevent irregular scan lines, and the flat surface ischosen so as to have a minimal amount of particle contamination. Byperforming a single master reference scan under favorable conditions,the possibility of variations between multiple reference scans due todiffering scan conditions is eliminated.

The structure is then placed on a linear stage and moved relative toatomic force microscope 10 with closed loop positioning (step 122). Inparticular, sensors may be positioned relative to or integrated with thestructure to provide signals to processor 22 related to the position ofthe structure relative to atomic force microscope 10. These signals maythen be used by processor 22 to assure that the structure is accuratelypositioned relative to atomic force microscope 10 in accordance with theprogrammed scan pattern.

After the structure has been scanned in accordance with the programmedpattern, the master reference scan is subtracted from the structure scanto correct curvature artifacts caused by the bowing effect in probe tip16 (step 124). In other words, because the bowing effect will cause thesame artifacts in the master reference scan and the structure scan, thecurvature in the structure scan can be substantially eliminated bysubtracting the master reference scan from the structure scan. Becausethe reference scan does not need to be performed after each structurescan, throughput of the scan process is improved. In addition, theclosed loop position feedback on the linear stage assures that the scanpattern is performed in the correct location in the scan area, thuslimiting curvature artifacts caused by positional offsets in the scanpattern.

In summary, a topographic profile of a structure is generated usingatomic force microscopy. The structure is scanned such that an area ofinterest of the structure is scanned at a higher resolution thanportions of the structure outside of the area of interest. An image ofthe structure is then generated based on the scan. To correct skew andtilt of the image, a first feature of the image is aligned with a firstaxis of a coordinate system. The image is then rotated to align a secondfeature of the image with a second axis of the coordinate system. Thestructure scan may be performed in a single scan or by integratingmultiple scans of the structure at different levels of resolution. Theresulting image of the structure has a higher resolution in the area ofinterest than in the remainder of scan area. This scan process preservesthe contextual details in the areas around the area of interest.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

The invention claimed is:
 1. A method for measuring topography, themethod comprising: scanning a surface of a structure to produce imagedata representing a profile of the surface including a first surfacefeature of the structure and also including a different second surfacefeature of the structure; manipulating the image data to align the firstsurface feature of the profile with a first axis of a first coordinatesystem; and manipulating the image data to align the second surfacefeature of the profile with a different second axis of the firstcoordinate system.
 2. The method of claim 1, wherein manipulating theimage data comprises: converting the image data of the profile from thefirst coordinate system to a different second coordinate system;offsetting the image data of the profile by a correction factor in thesecond coordinate system; and converting the offset image data of theprofile from the second coordinate system to the first coordinatesystem.
 3. The method of claim 2, wherein the first coordinate system isa Cartesian coordinate system and the second coordinate system is aspherical coordinate system.
 4. The method of claim 1, whereinmanipulating the image data comprises: rotating the profile about thefirst feature to align the second feature of the profile with the secondaxis of the first coordinate system.
 5. The method of claim 1, whereinmanipulating the image data comprises: aligning a surface feature of theprofile with a plane defined by the first axis and the second axis suchthat the surface feature is substantially parallel with the plane. 6.The method of claim 1, wherein the scanning step comprises: scanning anarea of interest of the structure at a higher resolution than portionsof the structure outside of the area of interest.
 7. The method of claim1 wherein further the scanning step is performed at a selected positionof the surface and the manipulating the image data to align the firstsurface characteristic step is performed without repositioning thesurface from the selected position.
 8. The method of claim 7 whereinfurther the manipulating the image data to align the second surfacecharacteristic step is performed without repositioning the surface fromthe selected position.
 9. A method for measuring topography, the methodcomprising: scanning a surface of a structure to produce image datarepresenting a profile of the surface; manipulating the image data toalign a first feature of the profile with a first axis of a firstcoordinate system; manipulating the image data to align a differentsecond feature of the profile with a different second axis of the firstcoordinate system; and manipulating the image data by converting theimage data of the profile from the first coordinate system to adifferent second coordinate system, offsetting the image data of theprofile by a correction factor in the second coordinate system, andconverting the image data of the profile from the second coordinatesystem to the first coordinate system.
 10. The method of claim 9,wherein the first coordinate system is a Cartesian coordinate system andthe second coordinate system is a spherical coordinate system.
 11. Anapparatus for measuring topography, the apparatus comprising: a probethat operably scans a surface of a structure to produce image data, theimage data representing a profile of the surface defining a firstsurface characteristic and a different second surface characteristic; aprocessor executing computer instructions stored in memory to manipulatethe image data to align the first surface characteristic with a firstaxis of a first coordinate system and to align the second surfacecharacteristic with a different second axis of the first coordinatesystem.
 12. The apparatus of claim 11, wherein the processor convertsthe image data of the profile from the first coordinate system to adifferent second coordinate system, offsets the image data of theprofile by a correction factor in the second coordinate system, andconverts the image data of the profile from the second coordinate systemto the first coordinate system.
 13. The apparatus of claim 12, whereinthe first coordinate system is a Cartesian coordinate system and thesecond coordinate system is a spherical coordinate system.